The goal of this project is to understand how membrane morphology is regulated by the septin cytoskeleton and by lipid composition. To study this question, we use the S. cerevisiae prospore membrane as a model. During sporulation, four new cells are generated within the mother cell as the prospore membrane grows to encapsulate the meiotic nuclei. Ultimately, the prospore membrane will act as the template for the deposition of the spore wall, and its inner leaflet will become the plasma membrane for the new haploid cells. Synthesis of the prospore membrane is essential for yeast to form functional, environmentally resistant spores. Our long-term goal is to understand the regulation of cellular architecture. The proposed research will contribute to our understanding of the molecular mechanisms important for shaping a cellular structure (the prospore membrane) important for the integrity and form of the yeast's meiotic products. The work in this proposal wil provide insights into fundamental aspects of cell biology concerning how cells and cellular structures acquire their shapes, and will also contribute to our understanding of septins, which are filament- forming proteins associated with membranes found in fungi and animals. In the previous funding period, the Spo71 protein was found to be important for determining the size of the prospore membrane and to be required for proper localization of septins along the prospore membrane. Preliminary studies suggest SPO71 is important in the elongation phase of prospore membrane development. The proposed experiments specifically examine the role of septins and septin regulators in prospore membrane development, and also examine how membrane lipid composition is regulated and contributes to prospore membrane morphogenesis. These studies will distinguish models for how these proteins work together to regulate PSM size and shape. As septins are an evolutionarily conserved protein family vital for a variety of cellular processes (including cell division, immune function, and neuronal development) and because the effect of membrane composition on membrane morphology is a fundamental issue in cell biology, the findings from these proposed studies should be of interest to a wide range of biologists.